1. Field of the Invention
This invention relates generally to heat exchangers and a method for manufacturing a heat exchanger, and more specifically relates to an internal heat exchanger for a free piston, Stirling cycle machine.
2. Description of the Related Art
Many machines require the transfer of heat from one mass to another such as transfer between a mass within the machine to a mass external of the machine. Free piston Stirling engines, heat pumps and coolers commonly require heat transfer both from outside its hermetically sealed pressure vessel, through the pressure vessel wall to the working gas at one location within the pressure vessel to provide a heat acceptor system and heat transfer from the gas within the machine at another location through the pressure vessel wall to a mass, such as a coolant, outside the pressure vessel to form a heat rejecter system. In order to improve the efficiency and rate of heat transfer, heat exchangers are commonly employed both interiorly and exteriorly of the Stirling machine's pressure vessel. An interior heat exchanger exchanges heat with the working gas in the machine's interior and conducts the heat to or from the pressure vessel wall. An exterior heat exchanger exchanges heat with an exterior heat source or a coolant, such as ambient air or a circulating coolant and conducts the heat to or from the pressure vessel wall. U.S. Pat. No. 4,052,854 to du Pré discusses heat transfer in a Stirling engine or heater.
U.S. Pat. No. 4,429,732 to Moscrip describes a regenerator, which is similar to a heat exchanger but stores heat and alternately transfers heat to and from the working gas and the mass of the regenerator as the working gas cycles through the regenerator. U.S. Pat. No. 5,373,634 to Lipp, although not for a Stirling machine, shows a heat exchanger having straight, open-ended passages with channels or orifices drilled into the sides of the structure transverse to the straight passages.
In the prior art, the larger Stirling machines usually resort to internal heat exchangers which are constructed of several parallel tubes conductively connected to the pressure vessel wall in order to increase the through-wall heat transfer surface area. However, such tubular heat exchangers require numerous braze joints for attaching the tubes to the wall. This large number of joints also greatly increases the probability of failure because of leakage and also increases the cost of fabrication.
Smaller Stirling machines commonly use a monolithic head construction where heat is transferred through the wall of the pressure vessel of the machine. When a monolithic head is used, it is common practice to braze an internal finned surface, often in the form of folded fins, to the head of the pressure vessel. Such heat exchangers have gas flow between parallel plates, where flow uniformity is extremely sensitive to the plate spacing because the mass flow rate is proportional to the cube of gap between the fins. Mass flow through the corners is therefore limited. The folded fins are fabricated from a sheet of material folded into multiple fins with passages between the fins. This process requires multiple steps of bending and forming, in addition to brazing the sheet components for connection to the head of the pressure vessel. Additionally, folded fins are not generally available in the spacing required by Stirling machines so they often require secondary annealing and resizing. Each of these fabrication steps adds further expense to the cost of the heat exchanger.
In addition to folded fins, radial fins have also been machined into a heat exchanger.
Therefore, it is an object and feature of the invention to provide an improved, more efficient and less expensively manufactured heat exchanger particularly for a Stirling machine.
Another object and feature of the invention is to provide a method for forming a heat exchanger at moderate cost that allows for efficient heat transfer.